Conventional machine interfaces generally require users to provide input mechanically, typically via one or more switches. The keys of a computer keyboard or the buttons on a computer mouse, for instance, are obviously switches. Even a tablet touch screen is at its core a collection of switches, arranged in an array that facilitates locating the point or region of touch based on the subset of switches that are being activated. Beyond just user interfaces for personal computers and consumer electronics, machine interfaces used in industrial, medical, and military contexts, such as robot teach pendants or input devices for computer-assisted surgery, likewise rely on contact-based control input via switches or similar mechanical input elements. Unfortunately, such reliance on physical contact and operation of switches severely limits not only the machine user's freedom to move about, but also the realm of types of user input that are possible. Therefore, an alternative user-interface approach that remedies the shortcomings of traditional interfaces by providing greater flexibility is needed.